Extreme ultraviolet (EUV) optical systems, developed for wavelengths in the 11–14 nm range hold great promise for applications in semiconductor lithography. Over time, the sophistication of EUV optical systems has increased, and many techniques developed for use at longer, optical and ultraviolet wavelengths are finding application in EUV technology. Among the promising new techniques being applied in the EUV are those that can control the wavefront of an EUV beam. Such devices include phase-shift masks for advanced lithography, special purpose gratings to be used, for example, in shearing interferometry, holographic optical elements for use as interferometric null elements, and diffusers for use in illumination systems.
Optical elements that operate in transmission have been used in EUV applications for many years. These elements most often consist of lithographically patterned absorber regions on very thin free-standing membranes. One-thousand to 2000-Å-thick silicon-nitride membranes are commonly used as the membrane material; nickel, cobalt, and gold, are often used as the absorber.
Achieving optimal utility from wavefront-modulating devices requires independent control of both the amplitude and the phase of the wavefront. Such control has yet to be demonstrated at EUV mostly due to the strong absorption that accompanies transmission through all solid materials, and hence, the lack of non-absorbing phase-shift materials.
Among known and available materials, molybdenum has a relatively high phase-shift-to-attenuation ratio. Recently, EUV imaging with phase-shift-enhanced masks has been demonstrated using patterned molybdenum masks. These demonstrations, however, have been limited to so-called attenuated phase shift masks due to the inherent absorption of molybdenum at EUV wavelengths. It would be highly desirable to fabricate strong-phase-shift masks, in which attenuation is balanced between the phase-shifting and the non-phase-shifting regions of the mask.
Optical system testing and characterization are areas of EUV optics in which at-wavelength testing has made great advances in recent years. Shearing interferometry has emerged as a promising option for future high-accuracy EUV interferometers. In ideal cases, shearing interferometry is performed by interfering two or more sheared, or displaced, copies of a wavefront under test. Several demonstrations of EUV shearing interferometry have incorporated binary amplitude gratings as the beam-splitting element. When small shear ratios are used, multiple-orders overlap causing potential confusion in the analysis. In principle, the zero-order beam could be eliminated using a phase grating, however, when the phase shift material also attenuates, as is the case with EUV light, a duty cycle (fraction of open area) other than 50% must be used. Deviation from 50% duty cycle causes the appearance of the even diffracted orders, which are absent from 50% duty cycle binary gratings. In this circumstance, to limit the presence or magnitude of unwanted grating orders, it would be desirable to fabricate a device in which the attenuation is evenly balanced between the phase-shifting and the non-phase-shifting regions of the device. In this way, a pure-phase grating could be produced, albeit with some spatially uniform absorption magnitude.
Interferometric testing of aspherical optical elements is often performed in the visible and ultraviolet wavelength ranges using well-characterized null elements to restore a spherical wavefront profile, modulating the wavefront phase with constant amplitude. At present, while no such analogous devices are used for EUV interferometry, the methodology is clear by extension from techniques used at longer wavelengths.
Diffusing elements play an important role in optical illumination systems. Such devices, however, are difficult to fabricate in transmission mode at EUV wavelengths due to strong absorption in phase-shifting materials. In order to generate the thinnest possible EUV transmission diffuser, the attenuation must be balanced at all phase shift values.